Bump notebook from 5.6.0 to 5.7.8

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 5.6.0 to 5.7.8. - [Release notes](https://github.com/jupyter/jupyterhub/releases) - [Changelog](https://github.com/jupyterhub/jupyterhub/blob/master/CHECKLIST-Release.md) - [Commits](https://github.com/jupyter/jupyterhub/commits) Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -33,7 +33,7 @@ matplotlib==2.2.2
 mistune==0.7.4
 nbconvert==5.1.1
 nbformat==4.4.0
-notebook==5.6.0
+notebook==5.7.8
 numpy==1.14.5
 packaging==16.8
 pandas==0.23.3